Low pressure gas, for example that produced by a refinery (such as refinery off gas) or an olefins plant, is generally composed of methane, hydrogen, ethane, ethylene, propane, propene, and heavier hydrocarbons. If recovered, the hydrocarbons are valuable product which otherwise would be lost with the low pressure gas in the plant's fuel gas system.
Refinery off-gas usually contains H2, CO, CO2, O2, CH4, C2H4, C2H6, C3H8, C3H6 together with some trace impurities such as such as oxygen, ammonia, nitriles, acetylenes, sulfur compounds, butadiene, chlorides, arsenic, mercury, and water in addition to acid gases H2S, CO2, and COS. These low pressure gases are produced from refinery units that manufacture conversion products such as hydrotreaters, alkylation units, fluid catalytic cracking units, platformers, etc. Valuable products including hydrogen, olefins, natural gas liquids (NGL) and higher Btu fuel gas can be recovered from the low pressure gas if an low pressure gas processing unit is installed.
Similar to refinery low pressure gas, the low pressure gas from olefins plants can also be processed to recover valuable products. The low pressure gas from olefins plants typically is richer in ethylene or propylene and the low pressure gas has different species of trace impurities from those in the refinery low pressure gas.
Other plants, as well, may produce low pressure gas with C2 and higher hydrocarbons, for which the method of the present invention may be useful in providing cost effective recovery of valuable C2 and heavier hydrocarbons.
Currently, these valuable hydrocarbons may be recovered from the low pressure gas by at least three different methods. A circulating lean oil process may be used to absorb propylene and heavier components from refinery low pressure gases. Although the absorption process provides a reasonable recovery of propylene and heavier components, it is energy intensive and requires several pieces of operating equipment. The amount of equipment needed generally leads to an increased quantity of control loops and the need for expensive plot space.
Cryogenic expander based technologies are increasingly used in preference to the lean oil absorption methods, because these technologies provide higher ethylene and ethane recoveries. A typical cryogenic expander based process involves a series of progressive cool-down steps in plate fin heat exchangers and vapor-liquid separation steps, followed by demethanization.
Currently, turbo expanders are used in combination with external refrigeration to increase the thermodynamic efficiency of the process, thus achieving higher percentages of natural gas liquids (“NGL”) recovery. The requirement of external direct refrigeration requires more equipment, controls, and instrumentation, as well as storage and handling of the refrigerant that is used. The storage of refrigerant also raises additional safety considerations due to these extra hydrocarbons being stored at the plant site.
Low pressure gas is usually available at a relatively low pressure of about eighty-five psia. To achieve higher NGL recoveries, the cryogenic expander based units require feed gas compression. Because low pressure gas is a complex mixture of hydrocarbons consisting of saturated and unsaturated paraffins, olefins, diolefins, aeromatics, and acid gas, the compression of low pressure gas is troublesome during operations. The low pressure gas composition is a mix of various gasses coming out of several different units. These units may operate at different capacities, and any one or more of them may not be operating at any particular time. Thus, an low pressure gas stream will vary appreciably in composition and flow rate depending on the source and the types of units operating at a particular time.
Generally, the compressors can be designed for a range of composition for the feed gas. However, it is difficult to predict the range of composition and flow fluctuation for the low pressure gas. Any change in composition outside the design range will result in reduced capacity or loss of recovery of NGL. Similar problems are faced in turbo expander operations. Moreover, if the content of heavier hydrocarbons increases in the low pressure gases then condensation of these hydrocarbons takes place at higher pressure in the upstream section, which if not recovered will result in loss of valuable NGL.
Various contaminants that appear in low pressure gas also cause mechanical and control problems for rotating machinery, resulting in sometimes frequent maintenance downtime and a resulting significant loss of revenue. The variations in low pressure gas feed stream molecular weights and flow characteristics also cause problems for turbo expanders used in low pressure gas processing, again often resulting in significant maintenance downtime. Similarly, unsteady operating conditions, solids formation, or thermal stresses can result in leakages in heat exchanger cores.
In an attempt to circumvent at least some of these problems, less efficient reciprocating compressors and other positive displacement machines are often used to compress the low pressure gas feed stream. However, it would be more desirable to process the inlet feed gas without compression.
A third method of recovering valuable hydrocarbons from low pressure gas is disclosed in U.S. patent application Ser. No. 12/730,424, which discloses a process that avoids the need to compress the inlet low pressure gas feed, as does the method of the present invention. However, the present method provides the flexibility of operating at higher temperatures to reduce the risk of solids formation, and can optionally be operated at lower temperatures if it is desirable to do so to increase yield.
Thus, it is desirable to provide an efficient process for low pressure gas processing that has good adaptability to the feed composition variation.
Another challenge for this recovery process is to keep the operating temperatures above certain levels to reduce the risk of blue oil formation, and the formation of hydrates and other solids. These warmer operating conditions make the process safe while still maintaining the higher recovery of valuable hydrocarbons. The reduced chances of solid formation reduce the need for maintenance while preventing long term equipment damage.
It is also an object of the invention to recover the valuable hydrocarbons (C2+) from low pressure gas without, or with minimal, compression of the feed gas.
It is a further object of this invention to extract the valuable hydrocarbons from low pressure gas by using as part of the apparatus a turbo expander for which the refrigerant is product, feed gas, reflux formed during an intermediate part of the process, or a mixture of two or more of these. Using these sources for the refrigerant eliminates the need for significant storage of a specific refrigerant type. Further, use of a turbo expander in the refrigerant loop also helps to startup the plant at reduced capacity, allowing the plant to generate the required refrigerant needed to attain the full capacity of the plant.
It is yet another object of the invention to efficiently recover ethane and ethylene from the low pressure gas in a cost effective process.